As is known, coordinate measuring machines generally comprise a bed designed to support the workpiece to be measured and a mobile unit for moving a measuring sensor with respect to the bed.
More in particular, the mobile unit generally comprises a first carriage that is mobile on the bed along guides parallel to a first axis, a second carriage carried by the first carriage and mobile along a second axis orthogonal to the first axis, and a third carriage carried by the second carriage and mobile with respect to this along a third axis orthogonal to the first two axes. The measuring sensor is carried by the third carriage.
In machines of the type briefly described above, the bed is normally made of granite, and has the dual purpose of supporting the workpiece and of defining the guides for the first carriage.
This involves a series of drawbacks.
In the first place, the positioning of the workpiece on the bed, particularly in the case where the weight of the workpiece is considerable, leads to a deformation of the bed itself, which causes a deformation of the guides and hence induces errors of measurement.
Furthermore, the movement of the carriages of the mobile unit along the guides, and in particular of the main carriage, induces deformations on the bed and hence alters the disposition of the workpiece. Further measurement errors are thus introduced.
Other drawbacks linked to the use of beds made of granite are constituted by the cost, weight, and difficulty of supply of the granite in a short time.
To solve at least partially the problems linked to the workpiece weight, solutions have been proposed in which the granite bed is decoupled from the supporting structure.
Illustrated in WO 89/03505 is a measuring machine comprising a metal base bearing the guides for the mobile unit, and resting on which is a workpiece table made of granite.
Illustrated in GB-A-2080954 is a measuring machine in which a workpiece table made of hard mineral material is constrained to an underlying metal base, provided with the guides for the mobile unit, via positioning elements without play and such as not to transmit stresses.
Both of the solutions described above call for complex and costly base structures and, in any case, use a granite table, with all the drawbacks that this entails.
WO 2009/139014 illustrates a coordinate measuring machine in which, in order to solve the problems referred to above, the bed comprises a perimetral metal frame provided with guides for the mobile unit and a workpiece table housed within the frame, in which the workpiece table and the frame are constrained to one another by constraint means of an isostatic type that decouple the deformations thereof.